Alcohol and nicotine dependence commonly co-occur and a variety of research suggests some of this co-morbidity may be due to overlapping genetic factors. Converging evidence from electrophysiology studies and rodent models of alcohol- and tobacco-related phenotypes supports the hypothesis that neuronal nicotinic receptors (nAChRs) may be a common site of action for these drugs. The nAChRs are ligand-gated ion channels containing a central cation pore that act as the primary targets for nicotine and the endogenous agonist acytelcholine. Alcohol appears to play a role in modulating the pharmacological properties of nicotine binding at nAChRs, usually by enhancing receptor function. Furthermore, several nicotinic receptor subtypes are known to be present on dopaminergic neurons and involved in mediating the release of dopamine in response to alcohol and nicotine. Through this mesolimbic dopaminergic pathway, these receptors (including the 13-7 and 22-4 subunits) may contribute to the rewarding properties associated with substance use. Recent work has provided evidence that several of the human nAChR subunit genes are associated with alcohol and nicotine behaviors in humans, including age of initiation, early subjective response, and dependence. This proposal will extend these human studies in three ways. First, the human genes for the 13-6 and 22-4 nAChR subunits will be resequenced using DNA samples from 100 individuals to identify novel variations. Second, multiple variations in these genes will be characterized in a sample of 4,146 individuals, for which DNA and alcohol and nicotine behavioral data have already been collected, to test for associations between specific DNA variations and these behaviors. Third, laboratory-based methods will be conducted to determine whether specific variations lead differences in gene expression using cell culture assays.